A subset of human acute leukemias with a decidedly unfavorable prognosis possess a chromosomal translocation involving the Mixed Lineage Leukemia (MLL, HRX, AU-1) gene on chromosome segment 11q23. The leukemic cells, which typically have a lymphoblastic morphology, have been classified as Acute Lymphoblastic Leukemia (ALL). However, unlike the majority of childhood ALL, the presence of the MLL translocations often results in an early relapse after chemotherapy. As MLL translocations are typically found in leukemias of infants and chemotherapy-induced leukemia, it has remained uncertain whether host related factors or tumor-intrinsic biological differences are responsible for the poor survival in patients with the translocations. Lymphoblastic leukemias with either rearranged or germline MLL are similar with respect to most morphological and histochemical characteristics. Immunophenotypic differences associated with lymphoblasts bearing an MLL translocation include the lack of the early lymphocyte antigen CD 10, expression of the proteoglycan NG2, and the propensity to co-express the myeloid antigens CD15 and CD65. This prompted the corresponding disease to be called Mixed Lineage Leukemia and suggested models, largely unresolved, in which the leukemia reflects disordered cell fate decisions or the transformation of a more multi-potential progenitor.
Generally, therapeutic treatment is more successful when tailored to the specific type of leukemia. Thus, a need exists for accurate and efficient methods for diagnosis of leukemia and identification of subclasses of leukemias.